Add archived newsletters

[MarcL]

PR #211 fixes an issue with incorrect newsletter URL formatting. As the data for this comes from the Email Octopus API, I can't change the HTML after it has been sent. It's currently fixed by some hacky URL changing at build time.

A better solution will be:

• retrieve the archived newsletters as a set of HTML data files
• alter the incorrect HTML once to avoid needing a build time transformation
• use 11ty's data flow to read the HTML files from disk at build time without the need to alter any HTML

We could convert the HTML to markdown files and then render markdown at build-time instead. That might also be a good solution.